EMI gaskets used in electronic equipment include braided wire sheath, metal finger stock, and conductive foam. Braided wire sheath may be used with or without a foam core, but in either case requires a high compression force. This is a disadvantage since the door or closure must force the gasket against the equipment frame to provide a seal against EMI leakage. The greater the compression force to provide an effective seal, the greater the stiffness and precision of the door or closure must be. This leads to increased manufacturing costs. An example of such a braided wire sheath seal is disclosed in U.S. Pat. No. 4,652,695, which discloses a wire mesh covered resilient core having projecting edge margins of conductive mesh material gripped by a continuously extending flange.
Metal finger stock also requires high to medium compression forces. Conducting foam or conductive coated fabric with a foam core requires medium compression force and suffers the additional disadvantage that it is susceptible to compression set, i.e. the seal is unable to spring back fully to its original shape after prolonged compression.
One kind of seal, for the door of a microwave oven, is disclosed in Canadian patent number 1164533, issued Mar. 27, 1984. The microwave oven door is constructed from a metal screen/glass plate sandwich. Before assembly, the peripheral portion of the metal screen is inclined or offset at an angle to the glass plane. During assembly of the door, the offset portion is forced flat by the door frame as the glass plate and screen are inserted into the door frame. This causes the central portion of the screen to bow away from the glass plate. When the door is closed, the cabinet frame surrounding the door opening bears against the bowed screen. Because the screen is spaced from the glass, it can deform to conform to the cabinet frame.
This arrangement would not necessarily be satisfactory for a larger cabinet, such as might house telecommunications equipment, because it still relies upon the door to provide the force to conform the seal to the cabinet frame. The larger doors are more likely to twist or warp, so the space between the door and the cabinet is liable to vary, reducing the effectiveness of the seal. Moreover, the microwave oven operates at a single, high, frequency and so need only provide capacitive coupling to seal between the door and the cabinet. In telecommunications equipment and the like, the seal must be effective for frequencies ranging from a few hundred kilohertz to several gigahertz. At the lower frequencies, resistive coupling is also needed, so a metal-to-metal contact is required. Oxidation increases contact resistance, necessitating either high pressure sealed contacts or arrangements for providing a wiping action as the door is opened or closed. Wiping action contacts are not entirely satisfactory, especially if the cabinet door need not be opened regularly.
Another shielding arrangement for enclosures housing electronic assemblies is disclosed in U.S. Pat. No. 3,969,572, which shows an EMI gasket comprising an elongated laminar strip of plastics foam and flexible plastic permanent magnet strip. An electrically conductive strip is wrapped spirally around the core, compressing the foam. The strip is attached to one of the members to be sealed and a strip of magnetic material is attached to the other. The attractive force between the magnet and the magnetic material urges the spiral strip into contact with the magnetic material. Although this provides electrical contact and hence effective sealing at the lower frequencies, the spaces between adjacent turns of the spiral strip would reduce the capacitive coupling and hence reduce high frequency shielding. Also, the sealing strip would be relatively costly to make and mount. The stiffness of the connection members which extend perpendicular to the contact portion would likely limit compliance.